Water resistance and moisture resistance are 2 different things. My Tru-Oil finish sheds water like a champ but in high humidity it doesn't stop moisture(humidity), nothing practical does. Maybe if you encased your bow in acrylic it wouldn't take on moisture but that is an impractical solution. What we try to do with a finish on a bow is to slow down the hygroscopic rate, the rate at which moisture is taken in or released depending on the relative humidity.
Jay Massey tipped over on a river in Alaska and lost one of his sinew backed, osage bows with a French Polish(linseed oil and shellac). A few days later he recovered the bow with little damage, moisture or otherwise. It is not water that is the problem for wooden bows(although I don't recommend soaking your bows in water) it is the relative humidity...and R/H takes as long going into wood as it does coming back out.
